The invention relates generally to a method of and an arrangement for copying colored originals.
More particularly, the invention relates to a method of and an arrangement for determining the amount of light required in each of the primary colors during copying of colored originals, especially frames of color film.
In a known procedure for determining the amount of light required in each of the primary colors during copying of a colored original, the transparency of the original in each of the primary colors is measured prior to copying. The measuring system employed for this purpose is designed in such a manner that the spectral sensitivity or response thereof in each color is identical to that of the copying material, that is, the material onto which the original is copied.
The requirement that the spectral sensitivities of the measuring system and the copying material be identical has been known for a relatively long time. See, for example, the West German Offenlegungsschrift No. 21 18 617. This requirement is based on the fact that a density increase in a color of the original or negative will not lead to a corresponding change in the amount of copying light if the spectral sensitivities of the measuring system and the copying material are different. Such color may then be improperly reproduced. The literature, however, indicates that it is very difficult to match the sensitivities of the measuring system and the copying material. See, for instance, page 284 of the publication by R. W. G. Hunt entitled "The Reproduction of Colour". The reason is that the applicable curves of transmissivity versus wavelength have very steep sides in the long wavelength region. An Example in the Hunt publication illustrates that too low a value for the color density of an original or negative is obtained when the measurement is performed somewhat away from the density maximum. The result is an undercorrection.
The concept of undercorrection and the associated concept of overcorrection are based upon calibration of a copier using a calibrating original or negative constituted by a specific type of film. The copier is calibrated by determining the amounts of light in the primary colors which yield an optimum copy of the calibrating original. When another original or negative constituted by the same type of film as the calibrating original is to be copied, the differences in the color densities of the originals are established. The amounts of copying light required to properly reproduce the second original are then determined by appropriate percentage changes in the amounts of copying light required for the calibrating original.
In order to calculate the amounts of light required to copy an original, the calibration values for the type of film constituting the original must be fed into the copier. The introduction of the calibration values into the copier creates certain problems in film processing plants. On the one hand, if the calibration values are to be supplied to the copier automatically, a great deal of organizational work is required during the preparation of orders. On the other hand, if the calibration values for a film are to be fed into the copier manually when the film enters the copier, a substantial amount of labor must be expended to supply the calibration values to the copier.
The calibration values for different types of film are generally different mainly because the color densities of the film and the densities of the masks used in manufacturing the film may vary from one manufacturer to another.
Another color copying procedure involves neutral gray compensation. This procedure does not require calibration values. Instead, the color densities of an original or negative are measured and, based on these measurements, the amounts of copying light are regulated to obtain a copy which, on average, has a neutral gray color composition. However, automatic regulation of the copying light when using the neutral gray principle leads to a color shift in the copy if the original has so-called color dominants, that is, intensively colored areas stemming from the motif of the image to be copied. Since such a color shift does not occur when a color predominates due to masking of the film or the presence of a color tone, employment of the neutral gray principle raises the problem of distinguishing between color dominants and excessive color induced by masking of the film or the presence of color tones.